Determine if we should have a presubmit for PyPy

As part of the repo consolidation project, the PyPy3.10 presubmit was removed from `proto-plus`. The reason is that 3.10 is no longer supported on the PyPy release page: https://downloads.python.org/pypy/

See https://github.com/googleapis/proto-plus-python/pull/507/changes/8be46a06d3297e521d690e7dfe012ec73692ebfc from https://github.com/googleapis/proto-plus-python/pull/507 where we needed a presubmit for PyPy to catch https://github.com/googleapis/proto-plus-python/issues/506.

As of writing only Python 3.11 is supported in PyPy. If newer runtimes are supported we may want to add presubmits for PyPy in gapic-generator-python and downstream in google-cloud-python.

Monitor https://downloads.python.org/pypy/ for new releases